1. Field of the Invention
The present invention is related to a method for controlling a parallel redundant power system composed of UPS modules coupled in parallel, and more particularly to a method that utilizes the internal impedance of the UPS modules in accompaniment with a current shift method to simulate a virtual inductor so that the drawbacks caused from the actual inductor are eliminated.
2. Description of Related Arts
With the almost total-dependence of modern societies on electricity for employment, living, communication and entertainment, any disruption to an electricity supply brings about a temporary halt to civilization as we know it. Especially in the present age where a breathtaking variety of electronic products are widely used, serious economic loss may possibly occur if a company loses critical data when the electricity supply is suddenly interrupted.
To solve the above problem, the uninterruptible power supply (UPS) is developed. However, because of the quick evolvement of the load specifications, a single UPS device may be unable to satisfy the electricity requirement, especially when the amount of the loads is increased. Therefore, it is a natural trend to design the UPSs as modules to perform a parallel redundant power supply system.
The control techniques for the parallel redundant power supply system are mainly categorized into two types, the wired-connected mode and connectionless mode. For example; U.S. Pat. No. 5,257,180 xe2x80x9cControlling system for parallel operation of AC output inverters with restrained crossed currentxe2x80x9d adopts the wire-connected mode. In another aspect, the connectionless mode is applied in U.S. Pat. No. 5,745,356 xe2x80x9cIndependent load sharing of AC power systems connected in parallelxe2x80x9d, U.S. Pat. No. 6,118,680 xe2x80x9cMethods and apparatus for load sharing between parallel inverters in an AC power supplyxe2x80x9d and U.S. Pat. No. 6,356,471 xe2x80x9cDynamic feedback adaptive control system and method for paralleling electric power source and an uninterruptible power supply including the samexe2x80x9d.
At present, many kinds of loads need the steady electricity supply for operating normally whereby strict control of the power quality in the parallel configuration is essential. One example of the connectionless mode being superior to the wire-connected mode is that the connectionless mode does not have the problem of system-level failure of single point failure, whereby the entire UPS system can achieve the highest reliability. In U.S. Pat. No. 5,745,356, only the DC energy is calculated and only the differentiation of the active power is concerned; however the reactive power is difficult to control. The differentiation manner in the prior art has some further drawbacks, such as the low anti-interference ability and being unable to process harmonic waves. In U.S. Pat. No. 6,118,680, the phase locking is determined by whether the voltage area is zero. Not only should the calculation accuracy be considered, but also whether the output voltage contains the harmonic wave must be considered. Further, with regard to the active power and the reactive power, U.S. Pat. No. 6,118,680 can not provide an efficient control manner. In U.S. Pat. No. 6,356,471, an inductor must be additionally coupled to the output of the power system in parallel.
The basic concept of the parallel connectionless operation is from the parallel connection of the power generators in the power system. However, there are still some different physical characteristics between the UPS and the power generator. The internal impedance of the generators represents as a large reactance, whereas on the contrary, the UPS has a small internal resistance.
With reference to FIG. 1, by simulating the power generator models to perform the parallel configuration of the UPS modules, the output terminal of each UPS module is coupled with a large inductor (Z1, Z2) in series.
The UPS module is simulated by an ideal voltage source {right arrow over (Voi)}=|{right arrow over (Voi)}| less than xcex4i and an equivalent impedance Zoi, wherein the impedance represents the resistance character. Further, Zsi=jXsi represents the output inductance of the xe2x80x9cithxe2x80x9d UPS module (i is an ordinal), and Zsi is much larger than Zoi, (Zsi greater than  greater than Zoi). If the internal impedance Zoi of the UPS module is ignored, the output power of the UPS module is calculated by the following equation:                               P          oi                =                                                            "LeftBracketingBar"                                                      V                    oi                                    →                                "RightBracketingBar"                            ·                              "LeftBracketingBar"                                                      V                    o                                    →                                "RightBracketingBar"                                                    X              si                                ⁢          sin          ⁢                      xe2x80x83                    ⁢                      δ            i                                              (        1        )                                          Q          oi                =                                                                              "LeftBracketingBar"                                                            V                      oi                                        →                                    "RightBracketingBar"                                ·                                  "LeftBracketingBar"                                                            V                      o                                        →                                    "RightBracketingBar"                                            ⁢              cos              ⁢                              xe2x80x83                            ⁢                              δ                i                                      -                                          "LeftBracketingBar"                                                      V                    o                                    →                                "RightBracketingBar"                            2                                            X            si                                              (        2        )            
According to the equation (1), the active power Poi is directly proportional to the phase angle xcex4i that is defined between the {right arrow over (Voi)} and {right arrow over (Vo)}. The reactive power is approximately directly proportional to the |{right arrow over (Vo)}| to represent the output voltage amplitude.
FIGS. 2A and 2B show a first line indicating the relationship between the active power and frequency and a second line indicating the relationship between the reactive power and voltage.
xcfx89=xcfx89oxe2x88x92kxcfx89*Pxe2x80x83xe2x80x83(3) 
V=Voxe2x88x92kV*Qxe2x80x83xe2x80x83(4) 
where xcfx89o can be set to 50 Hz or 60 Hz, and Vo can be set to 120Vac or 230Vac depended on the power output requirement.
Based on the equations (1) to (4), the parallel connectionless configuration is able to be established by the droop method.
With reference to FIG. 3, the relationship between the output voltage of each UPS module and the total output voltage of all UPS modules is shown by two vectors.
In FIG. 3, vector {right arrow over (Voi)} means the equivalent output voltage of the inverter of the ith UPS module (i is an ordinal), and {right arrow over (Vo)} represents the output voltage that is composed by all parallel connected UPS modules. In the condition that {right arrow over (Vo)} remains at a constant, when the frequency of {right arrow over (Voi)} is increasing, the phase angle xcex5i will accordingly increase. Further, the reactive power output from the ith UPS module also gets larger based on equation (1). According to equation (3) and the relationship of Pxe2x88x92xcfx89, the increase of the active power will cause the decrease in the frequency of {right arrow over (Voi)}, thus the phase angle xcex4i will be accordingly decreased. If the frequency of {right arrow over (Voi)} is decreased, the same result still will occur. Finally, the {right arrow over (Voi)} and {right arrow over (Vo)} will finally have the same frequency and the phase angle xcex4i between the {right arrow over (Voi)} and {right arrow over (Vo)} is keep at a constant.
With reference to FIG. 2B, the relationship between the reactive power and the voltage amplitude is shown. In the event of {right arrow over (Vo)} remains at a constant, when the output voltage of {right arrow over (Voi)} is increasing, the reactive power output from the UPS ith UPS module will accordingly increase based on equation (2). Further referring to the Qxe2x88x92V relationship in equation (4), the output voltage amplitude will then decrease. The balance relationship between {right arrow over (Voi)} and {right arrow over (Vo)} will ensure that both the amplitude of {right arrow over (Voi)} and {right arrow over (Vo)} can remain at a static status.
From the foregoing description, the droop method can be applied to accomplish the parallel connectionless operation on the premise that the output of each UPS module is coupled with a large inductor. Since the inductor is composed of windings, the entire size and weight of the UPS system will become extremely large and heavy if the inductor is coupled to the output of the UPS module. Moreover, when the load is coupled to the UPS system, the impedance of the inductor will interfere with the output voltage adjustment accuracy of the UPS system. Thus, one way to solve the problem is to remove the inductor from the UPS system. However, some necessary requirements such as the function of the parallel connectionless operation, the volume and weight of the UPS module, and the output adjustment accuracy etc. will be hard to be satisfied.
The main objective of the present invention is to provide a control method for a parallel power system to solve the problems caused from an actual inductor, such as its large size and heavy weight. The present invention is able to efficiently control both active power and reactive power. Moreover, because the present invention does not employ differentiation or integration to perform phase locking, the problem of harmonic wave is avoided. Further, even when the present invention is not coupled with an actual inductor to the output of the UPS, the droop method is still able to be achieved.
To accomplish the main objective, the method of the present invention comprises:
sensing an output current of a UPS module;
shifting a phase of the output current of the UPS module with an angle;
calculating an active power and a reactive power based on the shifted output current;
adjusting the frequency of the output voltage of the UPS module based on the Pxe2x88x92xcfx89 slope line; and
adjusting the amplitude of the output voltage of the UPS module based on the Qxe2x88x92V slope line.
A second objective of the present invention is to provide a parallel redundant power system being able to be operated without the actual control wires connected among UPS modules.
The connection means of the preferred embodiment of this invention is described as following is an example. The system is composed of multiple UPS modules connected in parallel, wherein each output of each UPS module is collectively connected to a power output distributor (POD) to form the parallel connection and then provide power to a load, wherein each UPS module further includes:
an inverter having an output terminal;
a PWM driving circuit to drive the inverter;
an inductor current detector coupled to the output terminal of the inverter;
an output voltage detector coupled to the output terminal of the inverter;
a load current detector coupled to the output terminal of the inverter; and
a control unit coupled to the PWM driving circuit, the inductor current detector, output voltage detector and the load current detector.
The control unit performs a current-shifting manner to make all UPS modules be connected in parallel.
The control unit is able to be accomplished by a digital signal processor (DSP) together with a software in the DSP.